The present invention relates to satisfying a plurality of gamma requirements for displaying images on a monitor, and in particular relates to processing images for a computer monitor such that images previewed on the monitor have the same appearance as they would have after being transferred to film and viewed in a theater.
Image transducers convert between patterns of light intensities and patterns of electrical charge. The relationship between light intensity at a point on an image and its corresponding electrical signal, expressed as a voltage, is non-linear. In a television camera, for example, a small increase in light intensity at high levels will result in a smaller change in voltage than the same increase in intensity at low light levels. A television monitor containing a cathode ray tube has the opposite characteristic: an increase in electron gun voltage has an exponential relationship with the intensity of light emitted by phosphors at the screen.
Each such transducer""s characteristic may be defined by a response curve, representing the transfer function of the transducer. For many transducers these response curves can be expressed mathematically as an exponent applied to a numerical argument representing the transducer""s input normalised to a range of zero to one. This exponent has come to be known in the image processing industry as gamma.
Historically, television cameras and monitors were designed to have compatible gamma characteristics. In early systems, the camera had a gamma of 0.4545, and the monitor had a gamma of about 2.8. This results in an overall gamma of 2.8xc3x970.4545=1.27.
The overall gamma of an image processing system is the exponent which broadly defines the relationship between the intensity of light reaching the viewer""s eye and the intensity of light that entered the camera. In a theoretically perfect system, an overall gamma value of 1.0 would reproduce image intensities exactly as they would be seen by the naked eye. However, it has been shown that images viewed artificially benefit from an overall gamma value of greater than one.
For television, the overall gamma value is usually 1.27, but overall gamma is used to characterise many types of image processing system, including film. For cinematographic film, the required overall gamma is 1.5. When images destined for film are being edited digitally it is necessary to preview the results of editing on a monitor prior to their conversion back to film. Unless compensation steps are taken, images will appear differently on the monitor than on the resulting film. This is mostly due to the difference in gamma characteristics between cinematographic film and CRT monitors.
In a typical digital editing environment, a film scanner digitizes film images that are subsequently transferred to a digital frame store. The digital frame store comprises an array of hard disk drives. A computer system accesses and manipulates the images in the frame store in real time. The resulting image data is supplied to a film recorder, which then transfers the images back onto film. In order for the correct effects and edit decisions to be made during manipulation of the image data, it is necessary to ensure that images viewed on a computer monitor appear as they will look in the resulting film.
It is an aim of the present invention to provide an improved method of modifying image data for display on a monitor, so that images displayed on the monitor are consistent with gamma requirements for film.
The present invention provides apparatus for processing image data, comprising instruction storage means, central processing means and graphical processing means. The graphical processing means is arranged to receive image data and display resulting images on a color monitor. The graphical processing means includes look-up means for transforming the gamma format of the image data supplied to it. The color monitor requires a first gamma transformation to satisfy calibration requirement and the image data requires the second gamma transformation to satisfy a compensation requirement. Instruction storage means includes instructions for the central processing means to perform processing steps, that comprise combining the first and second transformations to define transformation data in the look-up means and supplying images to the look-up means such that images displayed on the monitor are transformed in response to the second gamma transformation but not said first gamma transformation.